Ceramic article and method of making same



` Dec. 7, 1943. T, G Manou-GAL rAL' '2,336,182 CERAMIC ARTICLE ANUMETHOD of' MAKING SAME:

Filed May s1, 1940 enters Patented Dec. 7, 1943 2,336,182 y s CERADIIC ARTICLE AND METHOD F MAKING SAME Tainc G. McDougal and Karl Schwartzwalder,l Flint, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Dela- Ware Application May s1, 1940, serai N0. 338,094 12 claims. (Cl. 10s-s2) /This invention relates to ceramic bodies having compositions which lie in the ceramic oxide system BeO-MgO-AlzQs and to methods of making articles of such compositions. The preferred compositions are particularly adapted for use as insulators for spark plugs and the like which require one or more of the physical characteristics of high thermal conductivity, high thermal capacity, high thermal expansion, high dielectric strength at elevated temperatures and the ability to withstand sudden temperature changes. All of these physical characteristics are prerequisites contrastwith` 2000 C. required for BeO. In-

l creasing the percentage of MgO and A1203 lowers of satisfactory spark plug insulators for automo- BeO-MgO-Alzos and the four component system BeOe--MgO-AlzOa--SiOa produce very excellent insulators at temperatures only slightly higher than those required for ring presentnday commercial porcelaina The preferred combinations of oxides are disclosed in the triaxial diagram illustrated on the accom? panying drawing.

Referring to the figure the area A--D-E contains-the preferred spark plug insulator compositions in this system. This area corresponds to the following percentage range:

, Per cent BeO i 45-98 MgO 1-25 A1203 1-35 Bodies lying within this area'have the moet` desirable combination of the above-mentioned physical properties in combination with relatively low firing temperatures. Thus, .for example, point l on the diagram indicates a composition of 95% BeO, 2.5% MgO, 2.5% A1203 having a firing temperature of Orton cone 30 (1650 C.) in

oxide insulators withwhich We are familiar.

the sintering temperature. Thus, point 2 on the diagram indicates a composition of 90% BeO, 5% MgO, 5% A1203 having a sintering temperature of Orton cone 23 (1580 C.). cates a. composition of BeO, 20% MgO, 20% A120: having a sintering temperature of Orton cone 20 (1530 C.) which'is only slightly higher than that required for -conventional porcelain spark plug insulators.

Spark plug insulators made from these new compositions have such greater thermal conductivity and thermal capacity that, in measurements in terms of insulation length required for spa-rk plug insulators, they are six to seven insulation lengths better than the best grade of spark plug porcelain. That is, an insulator of the new composition 6 or rU32 of an inch longer than the best porcelain insulator will perform as well as such porcelain insulator with respect to the said thermal qualities. They are also superior in this respect to any of the increasingly used sintered At the same time no breakage occurs due to thermal shock andthe electrical resistance at high temperatures is considerably higher than that of porcelains now in general use. The thermal expansion is also greater, an advantage in that it more nearly approximates that of the metal hous ing in which the insulator is assembled to form the complete.compressiontight spark plug. It is indicated that these new compositions may provide as great improvement for such uses as radio work Where low power factor loss is necessary as they do for spark plug insulators.

The next best bodies have compositions lying within the area F-G-H-I on the diagram.

This area corresponds to. the following composition range:

v Per cent B a 15-40 Meo v p 15`4o A1203 l 45-65 Point 3 indiv designated by area J-K-L, corresponding to the following range of percentages:

. Per cent BeO 1-10 MgO 1-'10 A1203 38-98 The bodieswithin the three areas above specified are especially, adapted for use as spark plug insulators. For many other uses such as crucibles, and insulators other than for spark plugs, the same physical characteristics are not required. Very'useful bodies for such purposes will be found throughout the area designated A-L- X-W corresponding to the following percentage range:

Per cent BeO 1-98 MgO 1-40 A1203 1-98 The bodies within this larger area al1 possess high thermall conductivity, high temperature cined to form periclase; as prepared magnesium aluminate. When silica is to be present the magnesia can be added as magnesium silicates such as talc or magnesium aluminum silicates.

Alumina Alumina is preferably added as calcined alumina prepared by the Bauer process; as calcined diaspore or bauxite; as electric furnace or high temperature calcined alumina. When silica is to be present, clay, minerals of the sillimanite group, or other aluminum silicates, synthetic aluminum silicates, -,or alkali earth aluminum silicates, maybe added.

Silica Silica is preferably added as amorphous silica,

. quartz, cristobalite, or tridymite; in a combined the silica content below 20% although good insulators have been made with somewhat higher silica contents. i Y

The following are examples of the Quaternary bodies just discussed. A body consisting of 5% BeO, 20% MgO, 60% A1203, 15% S102 matures at Orton cone 20 (1530 C.), as compared with 1785a C. for the same body with the'silica omitted. At the same time the resistance of the body to thermal shock breakage is greatly improved.

The addition of 7.6% silica'to a body consisting of 39% BeO, 4% MgO, 57% 'A1203 reduces the sintering temperature from Orton cone (1785 C.) to'Orton cone 20 1530 C.) While maintaining the desirable physical characteristics.

The materials required for the bodies may be added as oxides or as various compounds or cornpositions having different mineralogical forms,

such as silicates, spinel, aluminatesand so on and it will be understood that where in the claims the bodies are dened by oxide content such variations are contemplated as equivalents. It will also be understood that small proportions of suitable known ceramic fluxes or diluent materials may be added when desired in accordance with Well-known practices in the ceramic art. The following are some examples of the forms in which the raw materials may be introduced into the batch:

Berg/Ilia Beryllia, preferably low in alkalies, may be added as the chemically precipitated beryllium oxide, raw or calcined; or as synthetic beryllium aluminate; or, when silica is to be present, as beryl or other beryllium containing minerals, or as a by-product from the treatment of beryllium minerals to obtain the metals.

Magnesz'a Magnesia is preferably added in a pure form such as 'electric furnace periclase; as magnesite or prepared magnesium carbonate; as natural or prepared hydrated magnesia, preferably calform, natural or' synthetic, such as beryllium, or

compositions in which clay is used to introduce A1203 and SiOz, by other methods well known to ceramic art, such as dry-pressing, puggng and grinding or turning. The usual organic plasticizers or inorganic gels or zeolites may be added to aid fabrication.

The raw materials employed in the body compositions should be in a very fine state of subdivision, preferably all the material finer than 43 microns with a greater portion lying between 0 and 5 microns.

In our preferred method of forming, disclosed in said Schwartzwalder patent, the inorganic materials are ground with la suitable proportion of temporary organic binder, such as Bakelite, t0-

' gether with a lubricant. The iinely ground material is then granulated and preformed into predetermined shapes. These shapes are then assembled on a center pin placed in the die and pressed into insulator shapes under heat and heavy. pressure. Setting of the binder produces a firm shapel readily handled in mass production facilitating the subsequent manufacturing operations. The formed body is red to a sufficiently high temperature to eliminate the organic binder and vto recrystallize the mass into a fine-grained dense impervious structure.

We claim:

l. A ceramic product made by firing a ceramic mixture showing upon chemical analysis approximately from 1 to 98% beryllium oxide, from 1 to 40% magnesium oxide and the balance alumina together with up to 20% silica.

2. An insulator for spark plugs and the like characterized by high thermal conductivity, high 4thermal capacity, high thermal expansion, high dielectric strength at elevated temperatures, high thermal shock resistance and good mechanical strength made by ring to a dense, non-porous lState, an insulator shape formed from a batch consisting of a finely pulverized mixture showing upon chemical analysis approximately from 45 to 98% beryllium oxide, from l to 25% magesium oxide and from 1 to 35% aluminum `ox- 3. An insulator for spark plugs and the like characterized by high thermal conductivity, high thermal capacity, high thermal expansion, high dielectric strength at elevated temperatures, high thermal shock resistance and good mechanical strength made by firing to a dense, non-porous state, an insulator shape :formed from a batch consisting of. a nely pulverized mixture showing upon chemical analysis .approximately from dielectric strength at elevated temperatures, high thermal shock resistance and good mechanical strength made by firing to a dense, non-porous state, an insulator shape formed from a batch consisting of a nely pulverized mixture showingupon chemical analysis approximately from l to 40% beryllium oxide, 15 to 40% magnesium oxide and from 45 to 65% aluminum oxide.

5. An insulator for spark plugs and the like characterized by high thermal conductivity, high thermal capacity, high thermal expansion, highv dielectric strength at elevated temperatures, high thermal shock resistance and good mechanical strength made by ring to a dense, non-porous state,an insulator shape formed fromabatchconsisting of a finely pulverizedmixture showinguponchemical analysis approximately from to 40% beryllium oxide, 15 to 40% magnesium oxide, from 45 to 65% aluminum oxide and up to 20% silica.

6. An insulator for spark plugs and the like characterized by high thermal conductivity, high thermal capacity, high thermal expansion, high dielectric strength at elevated temperatures and high thermal shock resistance and good mechanical strength made by ring to a dense, nonporous state, an insulator shape formed from a batch consisting of a finely pulverized mixture showing upon chemical analysis approximately from 1 to 10% beryllium oxide, from 1 to 10% magnesium oxide, from 88 to 98% aluminum` oxide and up to silica.

'7. A ceramic product made by tiring a ceramic mixture showing upon chemical analysis approximately from 1 to 98% beryllium oxide, from 1 to 40% magnesium oxide and the balance alumina together with 0 to 20% silica.

8. Ceramic product made by ring to dense, non-porous state a compacted body formed of a ceramic mixture showing upon chemical analysis approximately from 1 to 98% beryllium oxide, from 1 to 40% magnesium oxide and the balance alumina.

9. Ceramic product made by firing to dense,

' non-porous state a compacted body formed of a ceramic mixture showing upon chemical analysis approximately irom 1 to 98% beryllium oxide, from 1 to 40% magnesium oxide, up to 20% silica and the balance alumina.

10. An insulator for spark plugs and the like4 made by ring to a dense, non-porous state a. compacted insulator shape formed from a nely pulverized ceramic mixture showing upon chemical analysis approximately from 1 to 98% beryllium oxide, from 1 to 40% magnesium oxide, and the balance alumina.

11. The 'method of making spark plug insulators and the like which consists in preparing a nely pulverized ceramic mixture showing upon chemical analysis approximately from 1 to 98% beryllium oxide, from 1 to 40% magnesium oxide and the balance aluminum oxide, forming the mixture into the shape of the desired article, and thereafter ring the shape to sinter it into a dense, non-porous insulator body.

12. The method vof making spark plug insulators and the like which consists in preparing a nely pulverized ceramic mixture showing upon chemical analysis approximately from 1 to 98% `beryllium. oxide, lfrom 1 to 40% magnesium oxide, 

